The object of the present invention is a vibrator suited to emit very low-frequency vibrations and a method using this vibrator in order to explore a material medium with very low-frequency vibrations.
The vibrator according to the invention can be used in many fields where very low-frequency vibrations are required, notably for exploration of an medium by means of elastic waves, notably for seismic prospecting in geologic structures, or for structure tests, in civil engineering for example, etc.
Vibrators generally comprise a vibration generator of any type: electro-hydraulic, with eccentric weights, with a piezo-electric motor, etc., a reactive mass and a transmitter or coupling element such as a plate, a bell, a rod, etc., in contact with the medium where the waves are propagated, in order to exert thereon a constant force within a certain range of the frequency excursion. The motion of the coupling element is controlled by a pilot signal. The coupling element can be placed on the surface of the medium or in contact with the walls of a hole or of a cavity drilled in the medium.
The frequency excursion of the vibrator is limited on the low frequency side as well as on the high frequency side by certain technical contingencies. Concerning the upper limit, it is reduced on account of the resonance of the acoustic cavity formed by the plate and the first interface between layers of different impedances. It therefore depends on the structure of the medium. The practical limit in the ground is for example at frequencies of the order of 180 Hz for which the cavity has a sufficient quality coefficient to break the phase control of the plate. The motion of the plate being adjusted to exert a constant force, the displacement of the mass exerting this force by reaction on the plate is all the greater as the frequency is low. The lower limit of the possible frequency band is conditioned by the maximum travel of this mass.
A known method allowing to obtain elastic waves at a lower frequency is used in marine applications. It consists in using antennas referred to as parametric antennas by means of which very directional wave beams are emitted at close frequencies in the direction of a common target. Under the energetic action of the two beams, the target reacts in a non-linear way. The intermodulation signals of the two beams are radiated by the target and in particular the signal concerning the frequency difference. The solid medium is here at a distance from the sources of vibration. This method is described in patent FR-2,412,853 filed by the applicant.
Another known method of generating in a solid medium low-frequency elastic waves is for example implemented in patent FR-1,542,973. It essentially consists in causing the vibrations produced by two distinct vibrators to interfere with each other by means of a non-linear mixing element so as to obtain a lower beat frequency. The vibrators are for example vibrators with orbiting weights and the vibrations they generate are applied by means of resonant bars lying on a baseplate resting on the solid medium by means of the non-linear coupling element. This mixing element can for example be a rubber pad or plate, or a surface layer of the ground on which the baseplate rests.
This technique which consists in causing the vibrations of distinct seismic vibrators to interfere with each other actually gives interesting results only when the coupling elements (plates or bells) are nearly butt-jointed. This means that the part of the ground which is subjected to high stresses leading to non-linear operation does not significantly extend beyond the surface occupied by each coupling element. Measurements generally show that the ground depth which determines coupling of a vibrator is only of the order of some decimeters.
The vibrator according to the invention comprises a single vibration generator and at least one coupling element for transmitting the vibrations to the medium. It is characterized in that it comprises means for controlling the generator, suited to apply thereto (on each emission) a single pilot signal obtained by combination of at least two signals of different frequencies and non-linear elastic transmission means suited to generate in the medium vibrations at a frequency depending on the frequency difference of the two vibrational signals.
The non-linear elastic transmission means include for example a non-linear elastic transmission element interposed between the coupling element and the material medium, which can be made of a material showing a discontinuity in the deformation variation curve according to the compression applied or which can include at least one layer of the medium in contact with the coupling element.
The control means allowing to generate the pilot signal can comprise a fixed-frequency oscillator and an oscillator whose frequency can vary within a determined frequency range, or two variable-frequency oscillators whose frequencies vary within a determined frequency range, with a linear increase or decrease for example.
The control means can for example comprise a first oscillator whose frequency increases linearly within a first frequency range, and a second oscillator whose frequency decreases linearly within a second frequency range different from the first one, or two oscillators whose frequencies vary linearly in the same direction, with different slopes, within two distinct variation ranges.
The method of exploring a material medium by means of very low-frequency vibrations according to the invention comprises emission, by the vibrator, of vibrations in the medium, reception of the signals coming from the medium in response to the vibrations emitted and recording of the signals received.
It is characterized in that at least one composite pilot signal obtained by combination of at least two vibrational signals of different frequencies is applied to at least one vibrator in contact with the medium by means of a coupling element and the signals generated by the vibrator are applied by means of non-linear elastic transmission means (such as a non-linear elastic transmission element and/or a layer of the medium through which the vibrator applies the vibrations generated) so as to generate in the medium vibrations whose emission spectrum includes at least one frequency equal to the frequency difference of the two vibrational signals.
The frequency of one of the signals (or of the two signals) coming into the combination is preferably varied within a frequency band.
According to an embodiment, the vibrator is successively controlled by the first pilot signal and by a second pilot signal obtained by shifting the phases of the two vibrational signals by a quarter of a period.
According to an embodiment, the vibrator is successively controlled by a first pilot signal, a second pilot signal obtained by shifting the phases of the two vibrational signals by a quarter of a period and by two other pilot signals respectively phase-shifted by a half-period in relation to the first and to the second pilot signal.
The method comprises for example selecting from among said signals coming from the medium those which are relative to the frequency difference of the two combined signals, this selection being obtained by correlation with a synthetic signal.
A signal whose frequency is the difference of the two frequencies or depends thereon is for example used as the synthetic signal.
According to an embodiment, the medium where the vibrations are generated being a geologic formation, the elastic waves reflected by the discontinuities of the medium are picked up and combined with signals depending on the signals emitted in order to form a seismic image of the subsoil.